Heat-sensitive mimeograph stencils (hereinafter referred to as "stencils" for short) are known which comprise a thermoplastic film such as acrylonitrile-based film, polyester film, vinylidene chloride film or the like and a porous support such as a tissue paper, a non-woven fabric or a woven fabric, that is made of natural fibers, chemical fibers, synthetic fibers or mixtures thereof, the porous support being adhered to the above-mentioned thermoplastic film (for example, Japanese Laid-open Patent Application (Kokai) Nos. 51-2512, 51-2513 and 57-182495).
However, these conventional stencils have the following drawbacks.
(1) Since the film and the porous support are adhered by using an adhesive, the permeation of ink is hindered by the adhesive, so that clarity of image is low. PA1 (2) As for the adhesive used, for example, acrylic resin-based adhesives and vinyl acetate resin-based adhesives are likely to be softened, swelled and dissolved by printing ink, so that resistance to ink is poor. In case of using a thermosetting adhesive, since non-cured materials are likely to be left over, the stencil is likely to be fused with the thermal head during mimeographing. In the case of using a chlorine-based adhesive, toxic chlorine gas is liberated by heating the thermal head. PA1 (3) Further, in cases where an adhesive is used, an adhering step is necessary in the production process of the stencils, and equipment for recovering a solvent is required because a solvent is used when applying the adhesive. Thus, the cost of the production process is high. PA1 (4) Troubles such as breakage of the film and formation of wrinkles are likely to occur during the adhering step, so that the yield is low. PA1 (5) The adhesive and the solvent make the environment of workers bad. Use of adhesive and solvent is also not preferred in view of protection of the environment of the earth.
To improve these drawbacks, it has been proposed to make the amount of the adhesive used as small as possible (e.g., Japanese Laid-open Patent Application (Kokai) Nos. 58-147396 and 4-232790). However, the above-mentioned drawbacks have not yet been overcome completely.
As a method in which an adhesive is not used, Japanese Laid-open Patent Application (Kokai) No. 4-212891 proposes to form a heat-sensitive mimeograph stencil comprising a thermoplastic resin film and synthetic fibers scattered on one surface of the thermoplastic film, which are bonded to the film by thermocompression bonding. However, with this method, since synthetic fibers having lengths of not more than 50 mm are scattered by blowing wind or by electrostatic force, the fibers are not uniformly scattered, so that permeation of ink is irregular and clarity of image is insufficient. Further, since the adhesion between the resin film and the fiber layer is not necessarily sufficient, wrinkles and breakage are likely to occur during transportation of the film. It has been proposed to blend binder fibers in the fiber layer, or to apply a small amount of a viscose material to the film surface in order to attain complete adhesion. However, if binder fibers or a viscose material are used, permeation of ink is hindered and so clarity of the image is deteriorated. To overcome the drawbacks, it is desired to provide a heat-sensitive mimeograph stencil which does not utilize an adhesive, viscose material or a binder at all.
To promote clarity of image, it is desired that no fiber constituting the support exists at the region at which a hole is formed in order not to hinder permeation of ink. Therefore, it has been proposed to make the amount of the fibers constituting the porous support as small as possible, or to make the thickness of the fibers as small as possible (e.g., Japanese Laid-open Patent Application (Kokai) No. 59-16793). However, if the amount the thickness of the fibers is made small, the strength of the support is decreased, so that ease of transportation of the film is deteriorated. Further, since it is difficult to uniformly disperse the fibers, the basis weight and thickness of the support are irregular, so that permeation of ink is also irregular. As a result, clarity of image is deteriorated. In addition, when printed sheets are stacked, the image is transcribed to the backside of a sheet thereon.
To promote clarity of image and printing resistance, Japanese Laid-open Patent Application (Kokai) No. 2-107488 proposes a heat-sensitive mimeograph stencil comprising a thermoplastic resin film and a non-woven fabric made of synthetic fibers consisting of continuous filaments. However, the balance of retention and permeation of ink is not sufficient.